In drilling wellbores for oil and gas it is common to drill through the formation and subsequently to case the open bore with a liner or a casing (typically of metal) and to cement the liner or casing in place. Centralizers are used around the liner or casing in order to keep it in the middle of the borehole and to allow free flow of cement through the annulus between the casing and the wall of the borehole. This acts as a sealant and also as a mechanical support for the casing. Centralizers have therefore been adapted for attachment around the outer diameter of a liner or casing prior to the cement job. Centralizers can also be used to keep a screen in a central location in the wellbore as it passes through a formation.